1. Field of the Invention
The present invention generally relates to a mounting structure of electronic parts and, more particularly, to a mounting board to which electronic parts are mounted by using external mounting terminals made of a thermally-meltable joining material, a height-adjusting apparatus of such electronic parts, and a mounting method of such electronic parts.
2. Description of the Related Art
Many electronic parts such as semiconductor devices are mounted on a mounting board using mounting terminals (external connection terminals) that are made of a thermally-meltable bonding member such as a solder bump. For example, a BGA (hall grid array) type semiconductor device has many BGA bumps (solder balls) on a mounting surface, and the semiconductor device is mounted on a printed-circuit board by bonding the BGA bumps to electrode pads of the printed-circuit bard.
FIG. 1 is a cross-sectional view showing a state where a conventional BGA type semiconductor device is mounted to a printed circuit board. The semiconductor device 1 comprises a semiconductor chip 2, a package board 3 on which the semiconductor chip 2 is mounted, and a heat spreader 4 provided to cover the semiconductor chip 2. Many BGA bumps (solder ball bumps) 5 as external connection terminals are provided on a mounting surface of the package board 3. The semiconductor device 1 is mounted onto the printed circuit board 6 by heating and melting the BGA bumps 5 while causing the BGA bumps 5 to contact with electrode pads of the printed circuit board 6 and cooling the melted BGA bumps to be solidified.
With the improvement in a performance of electronic equipments, a number of input/output terminals of a semiconductor device is increasing every year. In connection with that, the size (package size) of the semiconductor device has become larger. In association with power on/off of electronic equipments, a temperature of a periphery of the semiconductor device is increased and decreased. If the package board 3 and the printed circuit board 6 have different coefficients of thermal expansion, plastic strain is generated in the BGA bumps 5 after mounting (terminals formed by the solder balls being melted and solidified) due to the difference between the coefficients of thermal expansion. The plastic strain increased with a number of times of power on/off, and if it exceeds a certain limit value, the bumps are destroyed, and, thus, the electronic equipment cannot be operated normally.
If the package size increases, the number of times of power on/off at which the plastic strain reaches the limit value is reduced, and, thus, the service life of the electronic equipment is shortened.
With respect to the decrease in the service life due to the above-mentioned bump destruction, it is known that the limit value of the bump destruction is increased and the service life can be prolonged if a height of the BGA bump is increased. In the conventional example shown in FIG. 1, the height of the BGA bumps 5 after mounting is determined by a surface tension of the solder. If an amount of solder of each BGA bump 5 is increased, the BGA bumps 5 can be higher, but the bump diameter is also increased, which results in a bump short-circuit being occurred easily due to adjacent bumps contacting with each other.
Moreover, in a semiconductor device generating a large amount of heat, a weight of a heat-radiating part such as a heat spreader or the like is increased, which results in an increase in a compression force applied to the BGA bumps. Even if the BGA bumps are extended to be higher, it may be deformed by a compression force due to a weight of the heat-radiating part and the height of the BGA bumps may be decreased.
Thus, as shown in FIG. 2, the height of the BGA bumps 5 can be changed by changing a height of a standoff 7 by providing the standoff 7 as a spacer between the semiconductor device 1 and the printed circuit bard 6. However, in this case, since the solder balls are melted after the standoff 7 (spacer) is placed, the solder balls attached to the semiconductor device 1, when solder reflow is being performed, may not reach electrode pads of the printed circuit board 6 if a distance between the semiconductor device 1 and the printed circuit board 6 (that is, a height of the spacer) exceeds a certain limit, which may cause a problem in that a so-called open failure occurs.
As a mounting structure similar to the mounting structure shown in FIG. 2, there is suggested a mounting structure that maintains a height of a leadless electronic part to be a constant height by providing a spacer between the leadless electronic part such as a chip resistor or a chip capacitor and a printed circuit board (for example, refer to Patent Document 1).
On the other hand, it is suggested that, in a mutual connecting process of a bard and anther board using solder, a height of solder bumps is increased by lifting one of the boards (separate from the other board) in a state where solder is melted (for example, refer to Patent Document 2).
Moreover, it is suggested that support columns formed of a shape-memory alloy are attached on a mounting surface of a semiconductor device, and the semiconductor device is separated slightly away from a mounting board by using an elongation of the support columns due to heat when solder reflow is performed so as to increase a height of solder bumps (for example, refer to Patent Document 3).
Patent Document 1: Japanese Laid-Open Patent Application No. 7-74450
Patent Document 2: Japanese Laid-Open Patent Application No. 11-111776
Patent Document 3: Japanese Laid-Open Patent Application No. 2000-150709